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Abstract:

The invention relates to scanning pulsed laser systems for optical
imaging. Coherent dual scanning laser systems (CDSL) are disclosed and
some applications thereof. Various alternatives for implementation are
illustrated, including highly integrated configurations. In at least one
embodiment a coherent dual scanning laser system (CDSL) includes two
passively modelocked fiber oscillators. The oscillators are configured to
operate at slightly different repetition rates, such that a difference
δfr in repetition rates is small compared to the values
fr1 and fr2 of the repetition rates of the oscillators. The
CDSL system also includes a non-linear frequency conversion section
optically connected to each oscillator. The section includes a non-linear
optical element generating a frequency converted spectral output having a
spectral bandwidth and a frequency comb comprising harmonics of the
oscillator repetition rates. A CDSL may be arranged in an imaging system
for one or more of optical imaging, microscopy, micro-spectroscopy and/or
THz imaging.

Claims:

1. A system for imaging in the THz spectral range comprising: a coherent
dual scanning laser system (CDSL) comprising two passively modelocked
fiber oscillators, said modelocked oscillators configured to operate at
slightly different repetition rates, such that a difference δfr in
repetition rates is small compared to the values fr1 and fr2 of the
repetition rates of said oscillators; a feedback system for stabilizing
the difference in the repetition rates of the two oscillators, wherein
information generated by said feedback system is used to generate a
frequency grid in the RF domain that has a 1:1 correspondence to a
frequency grid in the THz domain, and allowing an RF frequency to be
scale to a THz frequency; a material emitting THz radiation in response
to an output of said CDSL; and a detector responsive to said THz
radiation.

2. A system for imaging in the THz spectral range comprising: a coherent
dual scanning laser system (CDSL) comprising two passively modelocked
fiber oscillators, said modelocked oscillators configured to operate at
slightly different repetition rates, such that a difference δfr in
repetition rates is small compared to the values fr1 and fr2 of the
repetition rates of said oscillators; a monitoring system for monitoring
the difference in the repetition rates of the two oscillators, wherein
information generated by said monitoring system is used to generate a
frequency grid in the RF domain that has a 1:1 correspondence to a
frequency grid in the THz domain, and allowing an RF frequency to be
scale to a THz frequency; a material emitting THz radiation in response
to an output of said CDSL; and a detector responsive to said THz
radiation.

Description:

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a divisional of U.S. application Ser. No.
12/399,435 filed Mar. 6, 2009, the disclosure of which is incorporated
herein by reference in its entirety.

[0005] Dual pulsed laser systems comprising two modelocked lasers
operating at two slightly different repetition rates f1 and f2,
such that δ=|(f1-f2)|<<f1 and
δ=|(f1-f2)|<<f2, are useful instruments for
the rapid interrogation of optical response functions of widely varying
electronic and opto-electronic devices such as photoconductive switches
and integrated circuits. Additionally the use of dual pulsed laser
systems has also been suggested for THz imaging as disclosed in U.S. Pat.
No. 5,778,016 and U.S. Pat. No. 6,396,856 to Sucha et al.

[0006] The use of dual modelocked lasers can be replaced for probing the
optical response functions by implementing dual electronic circuit
systems, as has been suggested in U.S. Pat. No. 5,748,309 by van der
Weide. The approach has some benefit for the interrogation of the
spectral dependence of signal transmission in the THz spectral range. Two
pulsed signal sources also operating at two slightly different repetition
rates f1 and f2 were disclosed, which produce emission in the
THz spectral range made up of frequency lines comprising pure harmonics
of the two repetition rates. Detection of the beat signal at δ,
2δ, . . . n δ is then used to infer the signal transmission
at the harmonics of the repetition rate f1, 2f1, . . .
nf1. Note that in this scheme a beat signal at a difference
frequency as low as δ is used, which is not ideal, since δ
typically has a small value where acoustic noise can corrupt the signal.

[0007] The use of mode locked lasers was again later disclosed by Keilmann
et al., in `Time domain mid-infrared frequency-comb spectrometer`, Opt.
Lett., vol. 29, pp. 1542-1544 (2004), who suggested the use of a dual
scanning laser system for Fourier Transform Spectroscopy (FTS) and the
analysis of the spectral transmission of materials in the infrared
spectral range.

[0008] In order to improve the scan rate of dual laser scanning FTS,
Keilmann et al., in International Patent Application Publication
WO2007/045461, further suggested to dither the repetition rate of one
laser versus the other using techniques similar to the ones described in
the '016 patent.

[0009] The use of lasers for spectroscopy has also been suggested by
Haensch et al. in U.S. Pat. No. 7,203,402, where a single frequency comb
laser based on a mode-locked laser was used for the measurement of
certain properties of optical elements. Here the measurement was
performed either simultaneously or sequentially at each individual
frequency line of the comb laser.

[0012] In the following we refer to dual scanning laser systems that
exploit the discrete frequency spectrum, i.e. the comb spectrum, of
modelocked lasers but that do not require or do not rely on precision
comb control inside the laser oscillator as coherent dual scanning
lasers, CDSLs.

[0013] Here we disclose a new CDSL for applications in spectroscopy,
micro-spectroscopy, microscopy, Fourier transform spectroscopy (FTS),
optical and THz imaging, and/or similar applications. The CDSLs are based
on modelocked fiber lasers designed for operation at high repetition
rates allowing for large scanning speeds. Efficient spectroscopic
measurements are enabled by the implementation of low noise, phase
controlled fiber lasers, which are designed to provide broad spectral
coverage via the implementation of nonlinear spectrally broadening
optical elements. Various compact designs are described. In various
embodiments a reduction of component count is further accomplished via
simultaneous use of nonlinear spectral broadening elements and the use of
appropriate time delays between the lasers.

[0014] We further disclose the use of highly nonlinear waveguides in
conjunction with coherent supercontinuum generation for generating an
optical output from the visible to the mid-infrared spectral region.
Difference frequency generation (DFG) produces output in the mid-IR
spectral region and simplifies the implementation of FTS. DFG eliminates
variations of the carrier envelope offset frequency external to the laser
cavity and thus produces an output spectrum comprising true harmonics of
the laser repetition rates.

[0015] In conjunction with photoconductive antennas spectral emission in
the THz spectral region can be obtained.

[0016] In order to use difference frequency generation effectively, the
mode locked lasers can be configured with two outputs each. Amplifiers
can be further implemented to amplify those outputs. Supercontinuum
generation can then be implemented for spectral broadening of these fiber
laser outputs. Difference frequency generation can be implemented between
spectral components of the supercontinuum or between a spectral component
of the supercontinuum and another fiber laser output.

[0017] Nonlinear signal interference in nonlinear frequency broadening
elements from overlapping pulses can be eliminated by using separate
nonlinear frequency broadening elements for each laser. Alternatively, an
optical delay line can be inserted at the output of the CDSL to produce
an interference signal only from pulses that do not overlap in any
nonlinear optical elements. Electronic gating can further be implemented
for optimum signal conditioning.

[0018] In at least one embodiment the carrier envelope offset frequencies
in coherent dual scanning femtosecond modelocked fiber lasers can be
adjusted by control of various intra-cavity optical elements such as
intra-cavity loss, saturable absorber temperature, fiber temperature and
fiber grating temperature. In some embodiments carrier envelope offset
frequency control can be averted by the implementation of DFG.

[0019] In at least one embodiment the carrier envelope offset frequencies
and repetition rates in coherent dual scanning femtosecond modelocked
fiber lasers can further be controlled by phase locking the two lasers to
external cavities.

[0020] In at least one embodiment the carrier envelope offset frequencies
and repetition rates in coherent dual scanning femtosecond modelocked
fiber lasers can further be controlled by phase locking the two lasers to
two external single-frequency lasers.

[0021] In another embodiment the difference in carrier envelope offset
frequencies and the repetition rates in coherent dual scanning
femtosecond modelocked fiber lasers can further be controlled by phase
locking the two lasers to one external single-frequency laser.

[0022] For improved spectral resolution coherent dual scanning femtosecond
modelocked fiber lasers can also be constructed with lasers where the
repetition rate of one laser is an approximate harmonic of the repetition
rate of the other laser.

[0023] The noise of the carrier envelope offset frequencies can be
minimized by an appropriate adjustment of the intra-cavity laser
dispersion, and the pulse width injected into the supercontinuum fibers.

[0024] Any drift in carrier envelope offset frequency between the two
lasers in the CDSLs can be monitored and corrected for by external
optical means. Also, an f-2f interferometer can be implemented for
carrier envelope offset frequency monitoring.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] FIG. 1 is a diagram illustrating an example of a CDSL.

[0026]FIG. 2 is a schematic diagram of an optically integrated dispersion
compensator, and non-linear frequency conversion section as used for
supercontinuum generation.

[0027]FIG. 3 is a schematic diagram of a CDSL as used for optical imaging
applications.

[0028]FIG. 4 is a schematic diagram of a CDSL designed with a reduced
number of components.

[0029] FIG. 5 is a schematic diagram of yet another CDSL based on carrier
envelope offset frequency monitoring.

[0030]FIG. 6A is a schematic diagram of an intra-cavity assembly of a
modelocked fiber oscillator for resistive heating of an intra-cavity
fiber Bragg grating for carrier envelope offset frequency control.

[0031]FIG. 6B is a schematic diagram of an intra-cavity assembly of a
modelocked fiber oscillator for modulating the pressure applied to an
intra-cavity fiber Bragg grating for carrier envelope offset frequency
control.

[0032] FIG. 6c is a schematic diagram of an assembly including an
intra-cavity modulator as used for modulating the intra-cavity loss of a
modelocked laser for carrier envelope offset frequency control.

[0033]FIG. 6D is a schematic diagram of an intra-cavity assembly of a
modelocked fiber oscillator for modulating the residual pump power
impinging on an intra-cavity saturable absorber for carrier envelope
offset frequency control.

[0034]FIG. 7 is a plot of the RF spectrum of a carrier envelope offset
frequency locked Yb fiber laser operating at a repetition rate of 1 GHz
measured after a nonlinear f-2f interferometer.

[0035] FIG. 8 is a plot of the spectral output of Yb fiber laser based
coherent supercontinuum source operating at a repetition rate of 1 GHz.

[0036]FIG. 9 is a schematic diagram of a dual scanning laser system which
is locked to two external cavities for repetition rate and carrier phase
control.

[0037] FIG. 10 is a schematic diagram of a dual scanning laser system
which is locked to two narrow linewidth lasers for repetition rate and
carrier phase control.

[0038]FIG. 11 is a schematic diagram of an ultra-compact dual scanning
laser system which uses one external narrow linewidth laser for
repetition rate and carrier phase control.

DETAILED DESCRIPTION OF THE INVENTION

[0039] This description first discusses some aspects of mode-locked lasers
and frequency comb generation particularly related to CDSL and
applications thereof. Examples of applications of such lasers for IR
spectroscopy and THz imaging are included.

[0040] Modelocked lasers with fixed optical frequency spectra comprising a
set of equidistant optical frequency lines are typically also referred to
as frequency comb lasers. The optical frequency spectrum of a frequency
comb laser can be described by S(f)=fceo+mfrep, where m is an
integer, fceo is the carrier envelope offset frequency and frep
is the repetition rate of the laser. The amplitudes of the individual
frequency lines in fact sample the optical envelope spectrum at discrete
points fceo+mfrep in optical frequency space.

[0041] Frequency comb lasers were described in U.S. Pat. No. 6,785,303 to
Holzwarth et al., where the control of the pump power of a modelocked
laser in conjunction with electronic feedback loops is used to stabilize
fceo and thereby to stabilize the location of all individual
frequency lines that comprise the optical frequency spectrum. In a
standard modelocked laser fceo is not controlled and therefore only
the separation of all individual frequency lines is stable, apart from a
slow drift of fr due to cavity length fluctuations. As disclosed
above the spectral separation corresponds to the repetition rate
frep of the modelocked oscillator, which is generally a frequency in
the MHz regime, and in various embodiments described herein more
preferably about 1 GHz or even higher. The exact location of the lines
inside the frequency spectrum varies randomly. However, the optical
spectrum of a frequency comb laser and a modelocked laser can have the
same envelope function. Also even if fceo is not controlled, the
optical spectrum of a modelocked laser comprises a number of discrete
frequency lines.

[0042] When operating two frequency comb lasers at slightly different
repetition rates frep and frep+δ respectively, and when
overlapping the output of the two lasers on a detector various beat
frequencies can be observed in the RF domain.When further ensuring that
for both lasers frequency teeth of order m are the most proximate, the RF
spectrum includes harmonic frequencies mδ+Δfceo,
(m+1)δ+Δfceo, (m+2)δ+Δfceo . . . with
amplitudes from the geometrical mean of the amplitudes at optical
frequencies mf+fceo1 and m(f+δ)+fceo2 where
Δfceo=fceo1-fceo2. The parameter frep/δ
is the scaling factor that scales the RF frequencies to the optical
frequencies. For example for δ=10 Hz and frep=1 GHz, we obtain
a scaling factor of 108; an intensity measured in the RF domain at 1
MHz corresponds to an optical frequency of 1014 Hz. Δfceo
can be selected to further lower the RF frequency where measurements need
to be performed in order to obtain the amplitude of the optical
frequencies. The RF frequency at which measurements are to be performed
can be changed by ensuring that for the two lasers frequency teeth of
order m and n respectively are most proximate. In this case the beat
frequencies are given by (m-n)fr+mδ+Δfceo;
(m-n)fr+(m+1)δ+Δfceo;
(m-n)fr+(m+2)δ+Δfceo . . .

[0043] The need for a comb laser with a fixed optical frequency spectrum
for spectroscopic measurements can be relaxed via the implementation of
correction techniques that monitor the drift of the frequency lines
within the spectral envelope, as recently discussed by P. Giaccari et
al., `Active Fourier-transform spectroscopy combining the direct RF
beating of two fiber-based mode-locked lasers with a novel referencing
method`, Opt. Express., vol. 16, pp. 4347 (2008).

[0044] Alternatively, the drift of the individual frequency lines can be
eliminated by adding a nonlinear frequency conversion section after a
modelocked oscillator. For example when implementing difference frequency
generation between the red and blue part of the modelocked laser
spectrum, it is well known that the individual frequency lines occur
precisely at true harmonics of the laser repetition rate independent of
the value of fceo. As previously discussed, we refer to dual
scanning laser systems that take advantage of the discrete frequency
spectrum of modelocked lasers but that do not rely on precision phase or
fceo control inside the laser oscillator generally as CDSLs.

[0045] For any instrumentation applications of modelocked lasers,
mode-locked fiber lasers have several advantages over both mode-locked
bulk solid state lasers and mode-locked diode lasers. Mode-locked fiber
lasers offer typically superior noise properties compared to mode-locked
diode lasers and can be packaged in smaller spaces than mode-locked bulk
solid state lasers. Mode-locked fiber lasers can be produced with
excellent thermal and mechanical stability. Passively mode-locked fiber
lasers in particular can be constructed with few and inexpensive optical
components, suitable for mass production, as disclosed in U.S. Pat. No.
7,190,705 to Fermann et al. and Ser. No. 11/546,998 to Hartl et al. U.S.
Pat. No. 7,190,705 is hereby incorporated by reference in its entirety.
Additionally, the use of dual pulsed laser systems has also been
suggested in THz imaging as disclosed in U.S. Pat. No. 5,778,016 and U.S.
Pat. No. 6,396,856 to Sucha et al. The '016 and '856 patents also
disclose various techniques and configurations for controlling relative
and absolute timing drift of mode-locked lasers. U.S. Pat. Nos. 5,778,016
and 6,396,856 are hereby incorporated by reference in their entirety.

[0046] The dispersion compensated fiber lasers as disclosed in '705
provide for the construction of low noise frequency comb sources. Also
disclosed were designs of fiber lasers operating at repetition rates in
excess of 1 GHz.

[0047] Low-noise operation of fiber lasers minimizes their timing jitter,
allowing optimized control of the timing of the pulses. The '705 patent
disclosed the first low noise fiber-based frequency comb source. Low
noise operation was obtained by controlling the fiber cavity dispersion
in a certain well-defined range. Low noise operation of fiber frequency
comb sources is generally required in order to minimize the noise of the
carrier envelope offset frequency fceo of the laser to a negligible
level, and also to facilitate measurement and control of fceo.

[0048] Some examples of fiber-based CDSL systems are disclosed below.
Implementations providing for high repetition rate, low-noise, and a
high-level of integration are described. Non-linear spectral generation
and various implementations for phase-control lead to stable output
signals in the near-IR range, thereby providing benefits for IR
spectroscopy and THz imaging applications.

[0049] FIG. 1 schematically illustrates a coherent dual scanning laser
system 100 (CDSL) according to an embodiment. In this example two
mode-locked oscillators 110a, 110b having slightly different repetition
rates are utilized to provide input pulse trains. A pulse train from each
oscillator is amplified and split into first and second optical paths.
The pulses in each path are conditioned with a dispersion compensator. An
intermediate non-linear frequency conversion section generates a
supercontinuum along a first path, which is then combined with the pulse
train in the second path using a non-linear frequency converters for DFG
.The DFG outputs corresponding to each of the oscillators 110a, 110b are
then combined to produce CDSL output.

[0050] Referring to FIG. 1, the system comprises two oscillators 110a-,
110-b (oscillator 1 and oscillator 2), which preferably generate pulses
that can be compressed to the femtosecond (fs) time scale. Preferably
oscillators 110a,110b are implemented using Er, Yb or Tm oscillators
operating at repetition rates of about 250 MHz or higher. Such
oscillators were for example described in U.S. Pat. No. 7,190,705 to
Fermann et al. and Ser. No. 11/546,998 to Hartl et al. as well as in U.S.
provisional application U.S. 61/120,022 , entitled "Highly
Rare-Earth-Doped Optical Fibers for Fiber Lasers and Amplifiers" to Dong
et al., which is incorporated herein by reference. Various examples
disclosed in the '022 application include highly rare earth doped gain
fibers having pump absorption of up to about 5000 dB/m, and gain per unit
length in the range of 0.5-5 dB/cm. Various dopant concentrations reduce
Yb clustering thereby providing for high pump absorption, large gain,
with low photodarkening. Such rare-earth doped fibers provide for
construction of short cavity length fiber lasers, and for generation of
high energy ultrashort pulses at a repetition rate exceeding 1 GHz. Such
configurations provide for high signal to noise operation of CDSLs. By
way of example, other fiber configurations having high pump absorption
compared to conventional silica fibers, for example absorption of
300-1500 dB/m at 976 nm are also disclosed in U.S. Ser. No. 11/693,633,
entitled "Rare earth doped and large effective area optical fibers for
fiber lasers and amplifiers", now published as U.S. patent application
pub. no. 2008/0069508. U.S. Ser. No. 11/693,633 is hereby incorporated by
reference in its entirety.

[0051] The output of the oscillators is preferably passed through optical
isolators (not shown) to minimize their sensitivity to backreflections.
The repetition rates of the two oscillators can be monitored using two
tab-couplers inserted into the output of the two oscillators, which
direct a small fraction of the output of the oscillators onto two
detectors (not shown), which provide signals representative of the
repetition rate to controller 101.

[0052] The oscillators can be constructed to operate at respective
repetition rates of f and f+δ, where δ<<f.
Alternatively, the repetition rate of the second oscillator can be
selected to be nf+δ, where n is an integer. The difference between
their repetition rates δ, or (n-1)f+δ for the case of widely
dissimilar repetition rates, can then be controlled by repetition rate
control element 101 comprising phase-locked loops and an intra-cavity
transducer introduced into one of the oscillators. Such an intra-cavity
transducer can be a mirror mounted on a piezoelectric element or a fiber
heating element, for example as discussed in U.S. Pat. No. 7,190,705 to
Fermann et al. and U.S. patent application Ser. No. 11/546,998 to Hartl
et al. The oscillators may emit nearly chirp free pulses or slightly
chirped pulses. Preferably any chirped pulses emitted from the
oscillators 110-a,110-b have nearly-identical chirp. Preferably the power
of both oscillators can be adjusted over some range, for example by a
variable attenuator.

[0053] The outputs of the oscillators are coupled to two fiber amplifiers
120-a, 120-b. The fiber amplifiers are preferably cladding pumped. Such
cladding pumped amplifiers are discussed in U.S. Pat. No. 7,190,705 to
Fermann et al. Also cladding pumping via optical star-couplers as
described by Dong et al. in Ser. No. 61/120, 022 "Highly Rare-Earth-Doped
Optical Fibers for Fiber Lasers and Amplifiers" can be implemented and is
not further discussed here. Preferably the dispersion in both
oscillators-amplifier propagation paths is matched.

[0054] In the example of FIG. 1 optical signal pulses output from each
fiber amplifier are split into two arms: arms 125a-1, 125a-2 optically
connected to amplifier 120a and oscillator 110a, and arms 125b-1, 125b-2
optically connected to amplifier 120b and oscillator 110b. Optical fiber
couplers are preferred, and splitting ratios between 5/95 and 50/50 can
be implemented. Each arm may be implemented in an all-fiber configuration
as illustrated. In some embodiments at least a portion of an arm may be
constructed with bulk components.

[0055] Dispersion compensation is carried out in the optical paths of each
arm to compensate for dispersion, for example with a series of dispersion
compensating elements forming a dispersion compensator. At least a
portion of the arms may be constructed from identical components,
including the various dispersion compensating elements. Dispersion
compensating elements can include optical elements for pulse compression
to provide high-quality femtosecond pulses, and may provide complete
dispersion compensation or produce slightly negatively or positively
chirped pulses at their output. When complete dispersion compensation is
used, the output pulses are nearly transform limited.

[0056] A dispersion compensator can comprise several different fiber
elements, and may be implemented with an integrated "all-fiber" design as
will be further discussed below with respect to FIG. 2. For example, a
first fiber element can comprise a positive dispersion fiber designed to
spectrally broaden the output of the amplifier, and at least a second
fiber element for dispersion compensation and for compressing the
spectrally broadened output to near the bandwidth limit. Preferably the
pulse compressing fiber element comprises a dispersion compensating fiber
or a photonic crystal fiber with a central air-hole to minimize the
nonlinearity of the dispersion compensation stage. Pulse compression via
higher-order soliton compression in one or more negative dispersion fiber
elements can be implemented. Also, bulk optic dispersion compensation
elements such as grating, prism or grism pairs may be used. Preferably,
the pulses are compressed to a pulse width less than about 500 fs, more
preferably to a pulse width about less than 300 fs, and most preferably
to a pulse width less than about 100 fs.

[0057] Optical pulses propagating in arms 125a-2, 125b-2 are also
frequency converted in a non-linear frequency conversion section having
frequency conversion elements 130a, 130b. Frequency conversion elements
130a, 130b can include optical elements for supercontinuum generation to
provide pulses with a spectral bandwidth of at least a substantial
fraction of an optical octave, and substantially broader than an output
spectrum of oscillators 110a, 110b. Frequency conversion elements
130a,130b generate a broadband spectrum, for example a spectrum extending
into the near mid-IR region , for example extending from the near IR to
the range of at least about 3-5 μm, or up to about 10-20 μm.

[0058] In various implementations a frequency conversion section
preferably comprises a highly nonlinear fiber, a periodically poled
LiNbO3 (PPLN) waveguide, a silicon waveguide or any other suitable
nonlinear waveguide. An element may also be optically patterned or
periodically or aperiodically poled or have periodic variations of the
2nd-order nonlinearity along its length. The frequency conversion
sections 130a, 130b in each arm generate an optical supercontinuum
spectrum that can extend into the mid-IR when using highly nonlinear
fluoride or chalcogenide waveguides. Supercontinuum generation in
nonlinear waveguides was described in U.S. patent application 11/546,998
to Hartl et al. and is not further discussed here. As known in
supercontinuum generation, the fundamental frequency comb structure from
the oscillators is preserved. The additional spectral output generated
comprises individual frequency teeth separated in frequency by the
repetition rate of the laser. However, the injection of pulses shorter
than 300 fs, and more preferably shorter than 100 fs, reduces the
incoherent background between individual frequency teeth of the
supercontinuum spectrum. The incoherent background is undesirable because
it reduces the signal contrast in CDSLs. The influence of incoherent
background noise to the comb contrast is described in N. Newbury and W.
Swann, "Low-noise fiber-laser frequency combs," Journal of the Optical
Society of America B 24, 1756-1770 (2007) which is incorporated herein by
reference.

[0059] Difference frequency generation (DFG) is carried out in non-linear
frequency conversion sections 140a,140b by mixing the dispersion
compensated output signal pulses from paths 125a-1,125b-1 with the
corresponding dispersion compensated and frequency converted outputs,
including a supercontinuum generated in a section of arms 125a-2, 125b-2.
The outputs are mixed in frequency converters 140a, 140b. Frequency
converters 140a, 140b are preferably configured with nonlinear crystals
such as LiNbO3, GaAs, GaSe, GaP or any other suitable nonlinear
crystal. These nonlinear crystals may also be periodically poled,
optically patterned or have periodic variations of their 2nd order
nonlinearity along their length. Nonlinear waveguides can also be
implemented. Frequency filters and polarization controllers can further
be included upstream of DFG elements 140a, 140b and are not separately
shown. The output from DFG elements is combined via beam splitter 150 and
directed to the output 160.

[0060] In various embodiments the optical signal pulses output from the
amplifiers are further directed through an optical isolator before
injection into the dispersion compensation and frequency conversion
stages. Appropriate time delays between the two oscillators are further
introduced to ensure pulse overlap in the DFG elements and beam splitter
150. Such time delays can be introduced by well known methods of
controlling the fiber lengths and free space propagation paths and are
not separately shown here.

[0061] An all-fiber construction of a dispersion compensator and
non-linear frequency section of each arm provides some benefits. One
benefit of using a highly nonlinear fiber for frequency conversion
sections 130a, 130b is that the amplifier stages 120a, 120b, the
dispersion compensation elements and the frequency conversion section
130a, 130b can all be spliced together as schematically illustrated in
FIG. 2. Various elements are shown in FIG. 2 which may be used in each
arm, particularly in 125a-2,125-b2 where both dispersion compensation and
super-continuum generation are performed. Polarization maintaining fiber
components can also be implemented, or alternatively polarization
controllers (not shown) can be used to optimize the polarization state
for supercontinuum generation. A fiber pig-tailed isolator (not shown)
preferably isolates the output of the amplifier from unwanted
backreflections.

[0062] In the example illustrated in FIG. 2, a dispersion compensation
fiber 215 is spliced onto a length of transfer fiber 220 on each end,
which transforms the fundamental mode of the fiber to match the mode in
the adjacent fiber, such as any amplifiers 120a,120b providing inputs, or
a highly nonlinear fiber 230 providing an output as shown in FIG. 2. The
highly non-linear fiber, configured with the arrangement as shown in FIG.
2, may also be utilized in frequency conversion sections, for example
sections 130a, 130b. A transfer fiber 220 can comprise more than one
piece of fiber and can also comprise fiber optic tapers.

[0063] The output pulses of the amplifier emitted from transfer fiber 220
are then compressed in a length of dispersion compensation fiber 215. A
length of photonic crystal fiber can be used, but any other type of fiber
with suitable nonlinear and linear properties can also be implemented for
pulse compression. Both linear and nonlinear amplifiers, such as
similariton amplifiers as described in U.S. Pat. No. 6,885,683 to Fermann
et al., can be implemented. When nonlinear amplifiers are implemented the
oscillator power-levels can be preferably adjusted. For example
similariton amplifiers produce positively chirped pulses, which can be
compressed to near the bandwidth limit in a length of photonic crystal
fiber as discussed in U.S. Pat. No. 7,414,780 to Fermann et al.

[0064] The highly nonlinear fiber 230 is then used for supercontinuum
generation. Highly nonlinear fibers based on silica, are discussed in
U.S. Pat. No. 7,496,260, entitled "Ultra High Numerical Aperture Optical
Fibers" to Dong et al., which is hereby incorporated by reference in its
entirety. In various embodiments non-silica-fibers with improved IR
transparency can be used. For example nonlinear fluoride, bismuth,
telluride or chalcogenide fibers can be implemented. Such IR transmitting
fibers can transmit wavelengths up to around 20 μm and are
commercially available. Because the melting temperature of mid-IR
transmission fiber is typically much smaller than the melting temperature
of silica fibers, optical lens arrangements can further be used to couple
light from the dispersion compensating fiber to the highly nonlinear
fiber in order to avoid complicated splicing of fibers with largely
different melting temperatures.

[0065] An optical imaging system that includes a CDSL is shown in FIG. 3.
Here a beam splitter is inserted after the output of the CDSL and splits
the output along two optical paths. The beam splitter directs a fraction
of the output along a first path onto detector D2, which is used to
measure a reference spectrum, representing the output of the CDSL as a
function of wavelength. The sample under test is inserted into the second
path. By dividing the spectrum measured with detector D1 by the spectrum
measured with detector D2 an accurate absorption spectrum of the test
sample can be obtained. Such two detector schemes are well known in
standard Fourier transform spectroscopy to eliminate spectral variations
and temporal drifts of the light source in absorption measurements. In
order to obtain the spatial distribution of the sample absorption and to
perform imaging, an optical scanner 310 such as a commercially available
galvanometer mirror system is further inserted into the second beam path
of the output of the CDSL. In some embodiments the sample under test can
be mounted on a movable stage. In various embodiments a combination of
beam motion and movement of the sample may be utilized. The output of the
CDSL is then focused onto the sample under test with a microscope
objective 325 or other suitable beam delivery optics. The transmitted
light is detected with a detector D1. In various embodiments one
reference spectrum is obtained with detector D2. Alternatively, detector
D2 can be omitted and the reference spectrum can be obtained by taking
the sample out of the beam path 2. In some embodiments reflected light
may be detected, or a combination of transmitted and reflected light. In
order to improve the signal to noise ratio in the IR, cooling of the
detector can also be implemented. For example liquid nitrogen cooled
HgCdTe (MCT) detectors can be implemented that are commercially available
with detection bandwidths up to 100 MHz. Also filter wheels (not shown)
can be inserted anywhere in the beam path to select certain optical
frequency ranges. An image is then obtained by monitoring RF spectra for
each image point and by appropriately relating those RF spectra to
optical transmission or reflection spectra.

[0066] The detector D1 monitors beat frequencies in the RF domain. Due to
scaling of the optical frequencies to the RF frequencies with a scaling
factor frep/δ in a CDSL we can interpret the function of the
CDSL as representing a frequency grid in the RF domain for scaling RF to
optical frequencies;each optical frequency is uniquely mapped onto an RF
frequency, with a 1:1 correspondence. Difference frequency generation as
illustrated in FIG. 1 cancels the carrier envelope offset frequencies
external to the lasers. Because the carrier envelope offset frequencies
of the two lasers after the DFG stages are zero, the relation between
optical fopt and RF beat frequencies frf is given by

fopt=frf×frep/δ (1),

where the minimum RF frequency RFmin that contains information about
signal transmission at optical frequencies is given by mδ. Note
that since m is a large number (of order 104 or higher), RFmin
can be of the order of MHz.

[0067] An alternative embodiment of a CDSL is shown in FIG. 4. Here the
component count is reduced by using two oscillators operating at slightly
different repetition rates. The oscillator outputs are combined, and
coupled into a common propagation path. The components in the common
propagation path may be similar or identical to the ones described with
respect to FIG. 1. In this example one amplifier 420, one intermediate
supercontinuum generation section 430 and one DFG section 440 are used.
The output of amplifier 420 is split into arms 425a,425b in a manner
similar to that illustrated in FIG. 1. DFG is obtained from mixing the
dispersion compensated output of arm 425a with the output from the
supercontinuum generator configured in arm 425b. Non-linear crystal 440
provides for DFG, as discussed with respect to FIG. 1. The output of the
system is detected with detectors D1 and D2, where D2 is used to obtain a
reference spectrum and D1 is used to measure the absorption of the test
sample. Additional optical components for scanning can also be
incorporated as in the example of FIG. 3.

[0068] In order to avoid signal degradation due to nonlinear interactions
at times when the output pulses of the two oscillators overlap in time at
the DFG section, the detectors can be electronically gated to be
non-responsive at those times. In order to obtain an interference signal
at times when the output pulses of the two oscillators do not overlap in
time, an optical delay line 470 can be incorporated in front of the
detector D1 and D2 (or at the output of the CDSL) as shown. In at least
one embodiment a delay line based on a Mach-Zehnder interferometer is
utilized, although other types of delay lines such as a Michelson
interferometer with unbalanced arm lengths can also be implemented. The
time delay line conveniently produces a time delay of a fraction of the
cavity round trip time of the two lasers, where preferably this fraction
is 50%. When recording an interferogram with an optical delay line a
small penalty results from increased background signal and increased shot
noise, but that penalty is greatly offset by the benefit of reduced
component count of the system. Unwanted nonlinear pulse interactions due
to potential pulse overlap in other parts of the system can be further
avoided by an appropriate control of the pig-tail lengths from the two
oscillators.

[0069] Another example of a CDSL is shown in FIG. 5. As described with
respect to FIG. 1 the system also comprises a repetition rate controller
101, two oscillators 110a, 110b (oscillator 1 & oscillator 2) and
amplifiers 120a,120b. The system configuration is very similar to the
system described with respect FIG. 1, but the DFG sections are
eliminated. The output of the two oscillators propagates along two
different propagations paths and is injected into two separate fiber
optic amplifiers 520a,5200b. Preferably the amplifier and oscillator
exhibit overlapping gain spectra. Preferably both oscillators emit chirp
free pulses or pulses having nearly identical chirp. Preferably the
dispersion along the two propagation paths is matched. Preferably the
power of both oscillators can be adjusted over some range, for example by
a variable attenuator. The output of the amplifiers is further directed
through an optical isolator (not shown) before injection into two
dispersion compensators and frequency conversion sections similar in
design to the arms 125a-2, 125b-2 described with respect to FIG. 1. In
this example the frequency conversion sections generate two preferably
very broad supercontinuum spectra that can span an octave or more and can
extend into the mid-IR when, for example, using highly nonlinear fluoride
or chalcogenide waveguides. Several frequency conversion sections as well
as intermediate amplifiers can be concatenated and frequency conversion
stages based on PPLN waveguides or silicon waveguides can also be used.

[0070] In contrast to the system described with respect to FIG. 1, a
portion of the output of the two generated supercontinua is diverted to
two phase detection and control units 540a, 540b. Phase detection can for
example be conveniently performed with an f-2f interferometer as
discussed in U.S. Pat. No. 7,190,705 to Fermann et al. and Ser. No.
11/546,998 to Hartl et al. Such f-2f interferometers are therefore not
further discussed here. The f-2f interferometer produces an RF beat
signal corresponding to fceo which is fed back to the oscillators
for fceo control via a feedback loop. The fceo of both
oscillators can be kept within an RF filter bandwidth by a frequency lock
by feedback loops. For optimum precision of the feedback loop a phase
locked loop can be implemented, but other feedback loops can also be
implemented.

[0071] As discussed in U.S. Pat. No. 7,190,705 the temperature of an
intra-cavity fiber grating inside a modelocked fiber oscillator can be
used for carrier envelope phase control. Alternatively, as also discussed
in '705, an external pressure can be applied to the fiber grating and
pressure variations can be used for carrier envelope phase control.

[0072] In FIGS. 6 a variety of techniques for carrier envelope phase
control in a fiber oscillator are illustrated. In FIG. 6A a piece of
intra-cavity fiber 601 is shown that contains a fiber Bragg grating for
cavity dispersion control. In order to control the carrier-envelope
offset frequency, the outside of the fiber grating is gold coated and a
current is passed through the coating. The temperature of the grating can
thus be controlled via resistive heating in the gold coating which in
turn leads to a rapid modulation of the carrier-envelope offset
frequency, which can then be stabilized via a feedback loop in
conjunction with an f-2f interferometer.

[0073] In FIG. 6B a piezo-electric transducer (PZT) 602 applies pressure
to one side of the fiber, which can also be modulated and used for
intra-cavity carrier-envelope offset frequency control. The use of a PZT
allows for faster feedback control compared to a resistive heater.

[0074] Even faster carrier-envelope offset frequency control can be
accomplished via an intra-cavity acousto-optic modulator (AOM) 603 as
shown in FIG. 6c By changing the drive voltage to the AOM the loss inside
the fiber oscillator can be rapidly modulated, which in turn leads to a
modulation of the carrier-envelope offset frequency.

[0075] In FIG. 6D the carrier-envelope offset frequency is controlled via
a modulation of the residual pump power that is impinging on the
intra-cavity saturable absorber. This is accomplished by inserting a
polarizer in front of the saturable absorber and modulating the
polarization of the pump light 604. The polarization of the pump light
can be modulated in a variety of ways; essentially lossless and rapid
polarization modulation is possible by passing the pump light through a
length of polarization maintaining fiber which is coiled onto a PZT drum
and exciting both axes of the polarization maintaining fiber equally with
linearly polarized pump light.

[0076] Other means for carrier-envelope offset frequency control can also
be implemented; for example the temperature of the intra-cavity saturable
absorber can be modulated. Various combinations may also be implemented.
Moreover, the feedback systems of FIGS. 6 may also comprise multiple
feedback loops for independent measurement and control of the carrier
envelope offset frequency.

[0077] In FIG. 7 the corresponding RF spectrum of a carrier envelope
offset frequency locked Yb fiber laser operating at a repetition rate of
1.04 GHz measured after a nonlinear f-2f interferometer is shown. The RF
spectrum shows a peak at 1 GHz corresponding to the repetition rate of
the laser and two peaks at 210 and 830 MHz corresponding to the carrier
envelope offset frequency.

[0078] FIG. 8 illustrates an example of a supercontinuum spectrum
generated with a carrier-envelope phase locked Yb fiber laser operating
at a repetition rate of 1 GHz. Here the supercontinuum was generated in a
highly nonlinear optical fiber. The supercontinuum spectrum was recorded
from one propagation path of the fiber system as shown in FIG. 5.

[0079] Rather than controlling fceo with an f-2f interferometer,
fceo can also be controlled by referencing the frequency comb of a
mode locked laser to the Fabry-Perot resonances of a passive cavity. This
technique has some benefits: no octave spanning continuum generation is
required and relatively small power levels which can be provided by a
portion of the oscillator power are sufficient. This method is described
in R. Jason Jones and Jean-Claude Diels "Stabilization of Femtosecond
Lasers for Optical Frequency Metrology and Direct Optical to
RadioSynthesis" PRL 86, p. 3288 (2001) and in R. Jason Jones et al.
"Precision stabilization of femtosecond lasers to high-finesse optical
cavities". Phys. Rev. A 69, 051803 (2004) which is hereby incorporated by
reference in its entirety.

[0080] An embodiment utilizing external cavities is shown in FIG. 9. The
oscillator 110a, 110b outputs are combined and coupled into a single
propagation path, and amplified with fiber amplifier 920, as discussed
above. A portion of each oscillator output is also directed to reference
cavities 940a, 940b, and stabilized at two separated spectral regions to
a reference cavity. In this example two reference cavities 940a, 940b
with slightly different round trip times are shown. Both cavities are
preferably in close thermal and mechanical contact for all thermally and
mechanically induced fluctuations being in common mode. A configuration
with one external cavity is also possible. When using only one cavity a
birefringent element incorporated into the cavity provides for two
different round-trip times along two polarization axes, where those two
axes are in turn locked to each laser. Such an implementation is not
separately shown.

[0081] Gratings 950a,950b direct two spectral regions of the oscillator
output to two different detectors, which are then used to lock the two
different oscillator comb teeth to two different resonances of the
external cavities, which may be configured as passive cavities, or with
feedback control. With the use of one or two external cavities all four
degrees of freedom, namely fceo of both lasers, as well as frep
and δ are referenced to the cavity modes. In a preferred embodiment
a Pound Dreyer Hall scheme is used for locking the oscillators to the
reference cavities. The Pound Dreyer Hall scheme requires the
implementation of additional phase modulators (not shown) in the fiber
pig-tails that transport the signal to the external cavities. Instead of
separate phase modulators, phase modulation can also be implemented
intra-cavity by for example modulating one cavity end mirror. The Pound
Dreyer Hall scheme is well known in the state of the art for locking
mode-locked fiber lasers to external cavities and is not further
explained.

[0082] As an alternative to using passive cavities as stable references
for repetition rate and carrier phase control of CDSLs cw reference
lasers 1080a,1080b can be used as shown in FIG. 10. The oscillator 110a,
110b outputs are also combined and coupled into a single propagation
path, and amplified with fiber amplifier 1020, as discussed above.
Additionally, two stable, single frequency lasers are used for
stablization. Such single frequency lasers are preferably based on
semiconductor or fiber lasers and are commercially available. The
frequencies of both single frequency lasers are different and are
selected such that they both overlap with the spectra of the mode-locked
lasers, preferably at the low and high frequency parts of the spectra.
The frequency comb of each mode-locked laser can now be phase-locked at
two comb teeth to the stable cw-lasers by feedback control to fceo
and frep of both lasers, fixing two frequencies of each comb and
therefore stabilizing fceo and frep. When the stabilization is
arranged such that for each mode-locked laser a different number of comb
teeth are between the two fixed comb teeth, their repetition rates are
different which is required for CDSL. An implementation of this
stabilization method is described in I. Coddington, et al. "Coherent
Multiheterodyne Spectroscopy Using Stabilized Optical Frequency Combs,"
Phys. Rev. Lett. 100, 13902 (2008), which is included by reference.

[0083] A compact and highly integrated configuration of a CDSL is shown in
FIG. 11. The oscillator 110a, 110b outputs are combined and coupled into
a single propagation path, and amplified with fiber amplifier 120, as
discussed above. A portion of the amplified output is sampled and
directed to detector D1. Alternatively, the output of the oscillators or
the amplified outputs of the oscillators may be directed to two different
detectors, such an implementation is not separately shown. A single
frequency laser is used for repetition rate and carrier phase control. In
this scheme one of the frequency teeth of oscillator 110a is locked to
the single frequency laser via a phase-locked loop (PLL1) and one of the
frequency teeth of oscillator 110b is locked to the single frequency
laser via another phase-locked loop (PLL2). To ensure phase-locking the
cavity length of each laser can be modulated. After phase locking of
these two frequency teeth, the expression for the frequency spectra of
the two oscillators can be written as

mfrep+fceo1=fy+fb1 (2)

n(frep+δ)+fceo2=fy+fb2, (3)

where fy is the frequency of the single-frequency laser and
fb1, fb2 are the RF beat frequencies to which the frequency
teeth of the two oscillators are locked; δ is the difference
between the repetition rates of the lasers. Frep and δ can
further be locked to two more RF reference signals by controlling, for
example , the pump current to the lasers. We can further evaluate n and m
by setting for example the laser conditions such that
mfrep=fy+fb1, where fy+fb1 is obtained from an
external calibration using for example a wavemeter. The same procedure
can be implemented to obtain n. In the following we assume for simplicity
n=m. Taking the difference of eqs (2) and (3) we then obtain

nδ+Δfceo=Δfb. (4)

[0084] Since Δfb and δ are known and n, m can be a
obtained using the calibration procedure described above, from eqs. (2)
or (3), we can evaluate Δfceo to great precision.

[0085] It can then be easily shown that for Δfceo≠0 the
frequency grid in the RF domain is frequency shifted compared to eq. (1)
and the relation between optical fopt and RF frequencies frf is
modified as

fopt=(frf-Δfceo)frep/δ+fceo1.apprxe-
q.(frf-Δfceo)frep/δ, (5)

[0086] Here we can simplify eq. (5) since fceo<<fopt. In
this example Δfceo is stabilized rather than the individual
carrier envelope offset frequencies in order to obtain an accurate RF
frequency grid for the measurement of optical frequencies.

[0087] To obtain the best long-term precision for frequency measurements
with a CDSL system, both oscillators are preferably packaged in close
proximity in order to equalize any thermal fluctuations of laser
parameters between the two lasers. Also, the single frequency reference
laser is also preferably packaged with good thermal control.

[0088] Additionally, the system can be set up with amplifiers and
nonlinear frequency conversion sections for increased spectral coverage.
A temporal delay line 470 can be inserted in front of the detectors D2,D3
to detect pulse interference without temporal pulse overlap in the
nonlinear stages. Also, two detectors D2, D3 can be used, where one
detector is used for detection of a reference spectrum and the other is
used to determine the absorption characteristics of a sample.

[0089] The system of FIG. 11 is beneficial for commercial applications
because it can operate with a few components and a high level of optical
integration.

[0090] The embodiments described above may be combined in various ways to
produce alternative implementations. Many possibilities exist, and
various modifications may be made based on specific applications. For
example, a non-linear frequency conversion section may be configured with
at least one non-linear fiber amplifier to broaden a spectrum.

[0091] Referring back again to FIG. 5, a phase control unit can also be
replaced with a phase monitor unit. For example a phase monitor unit can
comprise a fiber optic tap splitter (inserted in front of the amplifier)
which diverts some of the oscillator light, and selects two narrow
spectral lines from the oscillator spectra to monitor the carrier
envelope offset frequency and repetition rate evolution. The carrier
envelope offset frequency evolution can also be monitored after the
amplifier or the first frequency conversion stage, but usage of the
oscillator signal provides for the lowest noise. Such phase monitor units
were discussed in Giaccari et al. and are not separately described here.
As an alternative to such a phase monitor unit also two f-2f
interferometers can be used which monitor the carrier envelope offset
frequency of the two oscillators. As discussed with respect to FIG. 3 the
output of the CDSL can be passed through an optical sample and can also
be directed to a detector, where beat frequencies in the RF domain are
observable. Due to scaling of the optical frequencies to the RF
frequencies with a scaling factor frep/δ in CDSLs, we can
interpret the function of the scanning dual laser system as providing a
frequency grid in the RF domain for scaling RF to optical frequencies,
i.e. each optical frequency is mapped onto a RF frequency. When locking
the carrier envelope offset frequencies of the two lasers to different
values eq. (5) can be used to obtain an accurate RF frequency grid for
the measurement of optical frequencies.

[0092] More complex modifications of the RF frequency grid and the
relation between optical and RF frequencies are obtained for small
continuous variations of Δfceo as well as δ. By the
implementation of a phase monitor unit the appropriate corrections of the
RF frequency grid can be calculated in order to obtain an accurate
conversion of RF to optical frequencies. Such corrections to the RF
frequency grid were discussed by Giaccari and are not further described
here. Similar corrections can also be applied when monitoring
Δfceo with an f-2f interferometer. Since the f-2f
interferometer allows a direct reading of fceo for of each
oscillator using RF techniques, Δfceo can be easily computed
and the optical frequencies can be calculated using equation (5).

[0093] The imaging arrangement as discussed with respect to FIG. 3 can
also be extended to the THz range. As discussed by Yasui et al., in Appl.
Phys. Lett., vol. 88, pp. 211104-1 to 3 (2006) a THz comb is generated by
a photo-conductive emitter excited by a femtosecond laser. The same
applies also when generating THz pulses via optical rectification in an
electro-optic crystal. Moreover, the frequency comb in the THz frequency
range comprises pure harmonics of the laser repetition rate. Thus two
slightly offset THz frequency combs can be generated by directing the
output of a CDSL system onto an electro-optic crystal or a
photo-conductive emitter. A system for generating THz frequency combs and
imaging in the THz spectral range can thus be constructed similar to the
implementation shown in FIGS. 3 and 5, where the frequency conversion
section and the phase control sections are omitted and the nonlinear
frequency conversion section is replaced with an electro-optic crystal
such as for example GaP, GaSe, periodically poled LiNbO3, optically
patterned GaAs or a photo-conductive antenna. Appropriate THz optics can
then be used for imaging the THz radiation onto a sample, which is
conveniently placed on a movable stage for optical scanning. In various
embodiments the scanner may be omitted, in part because of the present
limited availability of scanners suitable for use over the THz frequency
range. An appropriate detector such as a photo-conductive antenna can
then monitor the RF beat signal from which the THz spectrum can be
inferred using an RF analysis of the detected photo-current in the
detector as discussed by Yasui et al.

[0094] Thus the inventors have described CDSLs and some applications
thereof, and various alternatives for implementation including highly
integrated configurations.

[0095] In at least one embodiment a coherent dual scanning laser system
(CDSL) includes two passively modelocked fiber oscillators. The
oscillators are configured to operate at slightly different repetition
rates, such that a difference δfr in repetition rates is small
compared to the values fr1 and fr2 of the repetition rates of
the oscillators. The CDSL system also includes a non-linear frequency
conversion section optically connected to each oscillator. The section
includes a non-linear optical element generating a frequency converted
spectral output having a spectral bandwidth and a frequency comb
comprising harmonics of the oscillator repetition rates. In various
embodiments:

[0096] a frequency conversion section includes an output section that
receives and combines multiple input frequencies and generates a spectral
output at a difference frequency thereof, and the system includes an
intermediate non-linear frequency conversion section between at least one
oscillator and the output section, the intermediate section generating a
broadband spectrum having a bandwidth substantially greater than an
oscillator spectrum.

[0097] a CDSL is arranged in a measurement system that utilizes spectral
information, and a spectral output is utilized to probe a physical
property of a test sample with spectral components within the spectral
bandwidth.

[0098] a CDSL is arranged in an imaging system for one or more of optical
imaging, microscopy, micro-spectroscopy and/or THz imaging.

[0099] a CDSL based measurement system may include an element for optical
scanning.

[0100] a phase-locked loop controls the difference in repetition rates
between the oscillators.

[0101] an RF spectrum analyzer generates an output at RF frequencies
related to the optical frequencies with a conversion factor
(fr1+fr2)/2 δfr.

[0103] at least one fiber amplifier is included for amplifying one or more
oscillator outputs.

[0104] an integrated, all-fiber, dispersion compensator and non-linear
frequency conversion section is included, the integrated section
comprising one or more of a highly nonlinear fiber, a photonic crystal
fiber, a dispersion compensating fiber and/or a fiber having a central
air-hole.

[0105] a system includes a bulk optical element for dispersion
compensation, including at least one of a grating pair, prism pair and/or
grism pair, wherein dispersion compensation comprises pulse compression.

[0109] In at least one embodiment a coherent dual scanning laser system
includes two passively modelocked fiber oscillators. The oscillators are
configured to operate at slightly different repetition rates, such that a
difference δfr in repetition rates is small compared to the
values fr1 and fr2 of the repetition rates of two oscillators.
The CDSL also includes a non-linear frequency conversion section
optically connected to each oscillator, the section comprising a
non-linear optical element generating a frequency converted spectral
output having a spectral bandwidth and a frequency comb structure with a
frequency separation equivalent to the oscillator repetition rates. The
nonlinear frequency conversion section produces a spectral output
substantially broader than the spectral output from each oscillator.

[0110] In various embodiments:

[0111] a means for monitoring the difference in the carrier envelope
offset frequencies of the two lasers is included, wherein information
generated by the monitoring means provides a 1:1 correspondence between
RF frequencies and optical frequencies.

[0112] a correspondence is represented with a 1:1 mapping of said RF
frequencies to optical frequencies.

[0113] an f-2f interferometer is included for carrier envelope offset
frequency control of each laser.

[0114] a feedback system is included for stabilizing the difference in the
carrier envelope offset frequencies of the two oscillators.

[0115] carrier envelope offset frequency information generated by the
feedback system is used to generate a frequency grid in the RF domain
that has a one to one correspondence to a frequency grid in the optical
domain.

[0116] a feedback system includes a single-frequency reference laser.

[0117] two reference cavities are utilized for carrier envelope offset
frequency control of each oscillator.

[0118] one reference cavity is utilized for carrier envelope offset
frequency control of each oscillator.

[0119] two single-frequency reference lasers are utilized for carrier
envelope offset frequency control of each oscillator.

[0121] a ratio of a repetition rate to the difference in repetition rates
is at least about 106, and may be in the range of about 106 to
about 109.

[0122] The repetition rates fr1, fr2, and ratio of a repetition
rate to the difference in repetition rates are sufficiently high to
convert an RF frequency to an optical frequency.

[0123] At least one embodiment includes a system for imaging in the THz
spectral range. The imaging system includes a coherent dual scanning
laser system (CDSL) having two passively modelocked fiber oscillators.
The modelocked oscillators are configured to operate at slightly
different repetition rates, such that a difference δfr in
repetition rates is small compared to the values fr1 and fr2 of
the repetition rates of the oscillators. The system includes a material
emitting THz radiation in response to an output of said CDSL, and a
detector responsive to said THz radiation.

[0124] In at least one embodiment a coherent dual scanning laser system
includes two passively modelocked oscillators generating at least two
trains of short optical pulses. The oscillators are configured to operate
at slightly different repetition rates , such that a difference in
repetition rates δfr is small compared to the values fr1
and fr2 of the repetition rates of the oscillators. The system
includes a beam combiner for spatially combining trains of short optical
pulses to propagate along a common optical path downstream of the beam
combiner. A non-linear optical element is included for spectrally
broadening at least one train of said short optical pulses propagating
along the common optical path. A dual arm interferometer is configured
with different arm lengths so as to detect interference between pulse
trains when the pulses are not temporally overlapping in time prior to
entering the interferometer. In various embodiments an arm length
difference corresponds to approximately half the cavity round trip time
of said oscillators.

[0125] In at least one embodiment a coherent dual scanning laser system
includes two passively modelocked fiber oscillators generating two
separate trains of short optical pulses. The oscillators are adjusted to
operate at slightly different repetition rates, such that a difference
δfr in repetition rates is small compared to the values
fr1 and fr2 of the repetition rates of the oscillators. A
feedback system stabilizes the difference in the carrier envelope offset
frequencies of the two oscillators, and the feedback system includes a
single-frequency laser. A beam combiner spatially combines trains of
short optical pulses to propagate along a common optical path downstream
of the beam combiner. The system includes a non-linear optical element
for spectrally broadening at least one train of short optical pulses
propagating along a common optical path. A dual arm interferometer is
configured with different arm lengths so as to detect interference
between pulse trains when the pulses are not temporally overlapping in
time prior to entering the interferometer.

[0126] Thus, while only certain embodiments have been specifically
described herein, it will be apparent that numerous modifications may be
made thereto without departing from the spirit and scope of the
invention. Further, acronyms are used merely to enhance the readability
of the specification and claims. It should be noted that these acronyms
are not intended to lessen the generality of the terms used and they
should not be construed to restrict the scope of the claims to the
embodiments described therein.